Security Digest · Vulnerability record · Known exploited
CVE-2021-43226: Microsoft Windows Privilege Escalation Vulnerability
Exploited in the wild (CISA KEV). Windows Common Log File System Driver Elevation of Privilege Vulnerability
CISA KEV · added 2025-10-06 CVSS 7.8 High EPSS 3.3% · p88
Should you act?
Exploited: patch now if you run it
CISA has confirmed exploitation in the wild. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.
- Yes, if you run Microsoft Windows. CISA added this CVE to its Known Exploited Vulnerabilities catalog on 2025-10-06, which means there is reliable evidence of exploitation in the wild. US federal agencies must remediate by 2025-10-27; treat that as the outer limit, not the target. CISA also lists it as known to be used in ransomware campaigns.
- EPSS 3.3%, percentile 88: FIRST’s model estimates a 3.3% chance of exploitation activity in the next 30 days, higher than 88% of all scored CVEs (elevated). EPSS is a probability, not a verdict.
- Needs local access needed. Lower urgency for most teams unless an attacker could already run code on the machine.
What it is
Windows Common Log File System Driver Elevation of Privilege Vulnerability
CISA’s summary
Microsoft Windows Common Log File System Driver contains a privilege escalation vulnerability that could allow a local, privileged attacker to bypass certain security mechanisms.
Product: Microsoft Windows · Assigned by: microsoft · Published: 2021-12-15
Affected and fixed versions
| Vendor | Product | Affected versions (per CVE record) |
|---|---|---|
| Microsoft | Windows 10 Version 1809 | 10.0.0 < 10.0.17763.2366 |
| Microsoft | Windows Server 2019 | 10.0.0 < 10.0.17763.2366 |
| Microsoft | Windows Server 2019 (Server Core installation) | 10.0.0 < 10.0.17763.2366 |
| Microsoft | Windows 10 Version 1909 | 10.0.0 < 10.0.18363.1977 |
| Microsoft | Windows 10 Version 21H1 | 10.0.0 < 10.0.19043.1415 |
| Microsoft | Windows Server 2022 | 10.0.0 < 10.0.20348.405 |
| Microsoft | Windows 10 Version 2004 | 10.0.0 < 10.0.19041.1415 |
| Microsoft | Windows Server version 2004 | 10.0.0 < 10.0.19041.1415 |
| Microsoft | Windows 10 Version 20H2 | 10.0.0 < 10.0.19042.1415 |
| Microsoft | Windows Server version 20H2 | 10.0.0 < 10.0.19042.1415 |
Severity scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| microsoft | 3.1 | 7.8 High | CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H/E:U/RL:O/RC:C |
- Attack vector: local access needed
- Attack complexity: low
- Privileges required: low (any account)
- User interaction: none
- Scope: unchanged
CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.
Exploitation and urgency
- CISA KEV: added 2025-10-06; federal due date 2025-10-27. Known ransomware use: Known.
- CISA required action: Apply mitigations per vendor instructions, follow applicable BOD 22-01 guidance for cloud services, or discontinue use of the product if mitigations are unavailable.
- CISA SSVC assessment: exploitation active, automatable no, technical impact total.
- FIRST EPSS (model of 2026-10-21): 3.28% probability of exploitation activity in the next 30 days; percentile 88.
Timeline
- — CVE record published by microsoft
- — Added to CISA’s Known Exploited Vulnerabilities catalog
- — CISA remediation deadline for US federal agencies
- — CVE record last updated
Sources and references
Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.
Vendor and project advisories, patches
- Listed in CISA KEV notes (msrc.microsoft.com) msrc.microsoft.com
Vulnerability records
- CVE.org record CVE-2021-43226 cve.org
- NVD entry CVE-2021-43226 nvd.nist.gov
Government
Other references from the CVE record and advisory
- portal.msrc.microsoft.com portal.msrc.microsoft.com
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